What methods can be used to reduce or eliminate residual stress in stainless steel springs during design and manufacturing

Residual stress is a common and critical issue in the manufacturing process of stainless steel springs. It primarily stems from the uneven flow of the material during plastic deformation. When the stainless steel wire is bent and wound into the spring shape, the outer material is stretched while the inner material is compressed. This uneven deformation leads to the accumulation of internal stresses that persist even after the external force is removed.

Residual stress has a direct and significant negative impact on the performance of stainless steel springs. First, it reduces the spring's elastic limit, causing permanent deformation before reaching the design load. Second, residual stress significantly reduces fatigue life, causing the spring to fail prematurely after repeated loading cycles. More seriously, in certain corrosive environments, residual stress can become a trigger for stress corrosion cracking (SCC), leading to sudden brittle fracture. Therefore, effectively reducing or eliminating residual stress is critical to ensuring the high reliability and long life of stainless steel springs.

Heat treatment: The core technology for eliminating residual stress

Heat treatment is the most common and effective method for reducing or eliminating residual stress in stainless steel springs. The basic principle is to heat the spring to a specific temperature and hold it there, allowing the atoms within the material to gain sufficient energy to rearrange, thereby releasing and redistributing the stress caused by cold working.

1. Low-Tempering (Stress Relieving):

This is the most common stress relief heat treatment method. For martensitic stainless steels (such as 420 and 440°C) and austenitic stainless steels (such as 302 and 304), this is typically performed at a lower temperature.

Austenitic stainless steels (302, 304, and 316): The ideal stress relief tempering temperature is typically between 340°C and 450°C. Within this temperature range, the material does not undergo phase transformation, but the thermal motion of atoms is sufficient to release most internal stresses. High temperatures can cause carbides to precipitate at grain boundaries, reducing corrosion resistance, so strict temperature control is essential.

Martensitic stainless steels (410, 420, and 431): These springs are typically tempered after quenching, and temperature control is crucial. Stress relief tempering temperatures are typically between 250-400°C, effectively reducing residual stress while maintaining the required hardness and strength.

2. Solution Treatment and Aging:

For precipitation-hardening stainless steels (such as 17-7PH and 15-5PH), their ultimate strength depends on the aging treatment. Before forming, the wire is typically in solution, resulting in good ductility. After forming, aging not only allows the precipitation phase to enhance strength but also effectively eliminates residual stress. This process occurs simultaneously.

Mechanical Treatment: Improving Surface Properties and Stress Distribution

In addition to heat treatment, certain mechanical methods can also effectively improve the stress state of springs, particularly surface residual stress.

1. Shot Peening:

Shot peening involves the use of high-velocity jets of tiny steel or ceramic beads to impact the spring surface, creating a compressive stress layer.

Principle: The compressive stress generated by shot peening can offset the tensile residual stress on the surface. Since fatigue cracks typically initiate from the surface, this compressive stress layer can effectively hinder crack propagation, significantly improving the spring's fatigue life.

Application: Shot blasting is particularly suitable for springs subjected to high cyclic loads or extreme operating conditions, such as automotive engine valve springs and critical springs in the aerospace industry.

2. Prestressing:

Prestressing, also known as "compacting" or "setting," is a method for actively eliminating residual stress.

Principle: After the spring is manufactured, a compressive or torsional force exceeding its design load is applied to it, causing a slight permanent plastic deformation. This process redistributes stress within the spring, generating a residual stress in the opposite direction of the working load after the load is removed.

Effect: This reversed residual stress can offset some of the working stress, reducing the stress level in actual use, thereby improving the spring's load-bearing capacity and fatigue resistance.

Process Control and Material Selection

Controlling the generation of residual stress at the source is also crucial.

Choosing the Right Wire: Selecting high-quality, uniform stainless steel wire is essential. Improper cold drawing or cold rolling processes can introduce excessive internal stress.

Optimizing the forming process: Adjusting winding machine parameters, such as winding speed and feed rate, can achieve more uniform material deformation. Advanced CNC equipment can more precisely control the forming process, reducing uneven deformation.

Precise process control: From wire entering the factory to final heat treatment, strict process parameter control is required at every stage. For example, the heat treatment furnace's temperature uniformity, ramp-up and ramp-down rates, and holding time must be precisely monitored.